America is experiencing a generational shift away from military intervention. Those of us in our 50s and 60s witnessed the American defeat in Vietnam and the subsequent U.S. retreat from the world, followed by the Islamic revolution in Iran in 1979, the Soviet Union’s invasion of Afghanistan that same year, and the imposition of martial law in Poland from 1981-83.

 

President Ronald Reagan delivers his ‘tear down this wall’ speech at the Brandenburg Gate near the Berlin Wall, June 12, 1987. Agence France-Presse/Getty Images

We also witnessed the U.S. military revival under President Ronald Reagan, his willingness to confront the Soviet Union, and the U.S.S.R.’s subsequent collapse and an American victory in the Cold War. We saw the successful use of U.S. military power in Iraq in 1990-91 and in Kosovo and Serbia in 1994-99, and contrasted that with the genocidal results of inaction in Rwanda. America’s use of war to shape the world seems to many of us both necessary and proper.

Now consider how many Americans in their 20s, 30s and 40s view the world. The Cold War to them was unnecessary—a tense and massively expensive arms race for little if any gain. The minor triumphs of the 1990s to them seem unimportant and related somehow to what is uppermost in their minds: the long and painful failures in Iraq and Afghanistan.

Al Qaeda killed Americans on 9/11, so killing Osama bin Laden was justified, but the U.S. did not have to wage wars in Iraq and Afghanistan to do that. China and Russia suppress democratic reforms and bully their neighbors, but how will military force help anything?

These younger Americans don’t suffer from moral indifference. They can see evil in Vladimir Putin‘s Russia, Bashar Assad’s Syrian regime, North Korea’s dictatorship, the Taliban and China’s anti-democratic Communist leadership. They understand that there is persecution of Christians and Muslims, Tibetans, women, gays and democracy activists abroad. They just cannot believe that the use of U.S. military power will make things better.

The task, therefore, isn’t to convince them that they must support military action when they believe in their hearts that it cannot work. The task is to demonstrate where there are still dangers in the world that threaten America directly and where we must be willing to use force to reduce them, and where the threats are indirect and means other than the use of force are appropriate.

What threatens America directly? The ability to build nuclear weapons and the means to deliver them is spreading. Iran, North Korea and Pakistan have gone to great expense to acquire long-range missiles and nuclear weapons. Saudi Arabia purchased small numbers of long-range missiles from China in the 1980s that have little value without nuclear warheads, and in 2009 King Abdullah stated clearly that if Iran crosses the nuclear threshold, “we will get nuclear weapons.” Russia has thousands of tactical nuclear weapons and has a doctrine that justifies their first use.

A world in which more countries have nuclear weapons is a world in which the chance of nuclear warheads being launched or falling into the hands of terrorists is greater. We must be able to prevent the conventional or unconventional use of nuclear weapons against the U.S. by whatever means necessary. Our government should launch a new Manhattan Project to improve our ability to detect and track the movement of nuclear weapons around the globe. In the worst case, American military capabilities—on land, at sea and in the air—will be needed to deal with this ultimate threat to the safety of Americans.

As for indirect threats, China and Russia have shown themselves willing to use economic and military might to intimidate and coerce their neighbors. The old realist argument was that we should not allow a hostile power to gain control of most of Eurasia because a power that did so would be strong enough to conquer us. Today, the idea of Chinese soldiers occupying Japanese or Indian territories seems fantastic, as did the idea of Russian invading its neighbors to the west just a few weeks ago. That does not mean we have nothing to worry about.

It is important for Americans to understand that human progress has come through the competition of new ideas: religious, artistic, scientific and economic. Diversity of ideas and their free expression is the prerequisite of that competition. Tyrannies suppress competing ideas. This is not simply a problem for those who live under them. Tyrannies are prone to use military coercion, maritime coercion, cyber-coercion, energy coercion and resource coercion to make other nations toe the line and accept their version of borders, history and human rights.

The U.S. is not exempt from this coercion. Americans should help independent countries defend their autonomy because we benefit from their creation of wealth and ideas but also because we do not want to be coerced ourselves. We do this through military alliances, energy cooperation and trade alliances, collective cyber-defenses, and military cooperation and the sale of advanced weapons systems.

In short, Americans must help others defend themselves against tyranny because it is in our national interest to do so. That, no matter what generation one comes from, has not changed.

Mr. Rosen is a professor of national security and military affairs at Harvard.